At the same time, Dr. Scott Gottlieb, the agency’s commissioner, suggested that Congress strengthen the FDA’s authority over an estimated $40 billion industry, which sells as many as 80,000 kinds of powders and pills with little federal scrutiny.

These products range from benign substances like vitamin C or fish oil, to more risky mineral, herbal and botanical concoctions that can be fatal.

“People haven’t wanted to touch this framework or address this space in, really, decades, and I think it’s time we do it,” Gottlieb said. He is particularly concerned about supplements that purport to cure diseases for which consumers should seek medical attention.

“We know there are effective therapies that can help patients with Alzheimer’s,” he said. “But unproven supplements that claim to treat the disease but offer no benefits can prevent patients from seeking otherwise effective care.”

The companies included TEK Naturals, Pure Nootropics and Sovereign Laboratories. In their letter to TEK Naturals, the FDA and the Federal Trade Commission chastised the company for marketing “Mind Ignite” as a product “clinically shown to help diseases of the brain such as Alzheimer’s and even dementia.”

In response to a request for comment, TEK Naturals said in an email that it was reviewing the agency’s letter and was committed to complying with legal requirements.

Dr. Peter Lurie, president of the Center for Science in the Public Interest, a nonprofit advocacy organization, said the FDA’s warning letters call attention to the lack of regulation.

“Each time they issue a new one, it hopefully acts as a shot across the bow for that segment of the industry,” said Lurie, a former associate commissioner with the agency.

He also expressed frustration that the FDA has spent years debating policy that would encourage manufacturers to notify the agency of new dietary ingredients but has not yet released it.

Gottlieb said the FDA would create an online watch list of specific ingredients the agency is concerned about.

The FDA’s oversight is based on a 1994 federal law that imposed minimal reporting and labeling requirements on the makers of vitamins, minerals and herbs — a fledgling industry at the time. To prevent a company from selling a product, the law requires the FDA to prove that it is unsafe. The agency says that it is difficult to track the myriad products, many of which are sold on the internet.

The law also requires businesses to notify the FDA that they are making a dietary supplement, but not to say what’s in it.

“That just makes it impossible for the agency to police the market,” Lurie said.

The 25-year-old law has drawn increasing criticism as the sector has grown. There are now 50,000 to 80,000 dietary supplements on the market, according to the FDA. The agency also says that three of every four American consumers now take a dietary supplement regularly. For older Americans, the rate is four out of five.

In recent years, the FDA has cracked down on several sectors of the industry, including dietary supplements sold for weight loss, sexual function and energy enhancement. More recently, one of the biggest targets has been kratom, a botanical substance that is marketed as a safe alternative for opioids or to help opioid users’ withdrawal symptoms. The FDA has ordered that kratom imports be seized and told companies not to use it in supplements. The agency has linked several deaths to kratom.

Steve Mister, chief executive of the Council for Responsible Nutrition, which represents the dietary supplement business, said he believed the dietary-supplements industry is “remarkably safe.” He also said the current law strikes the right balance between ensuring safety and giving consumers access to affordable products.

“These products are not drugs and should not be regulated like drugs,” he said. Mister also said his group supports the FDA’s enforcement actions and has been working with Congress to increase the program’s budget.

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